linux-kernel - Re: [PATCHv3 16/32] x86/boot: Allow to hook up alternative port I/O helpers

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Date:   Tue, 22 Feb 2022 01:02:33 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...el.com, luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 16/32] x86/boot: Allow to hook up alternative port I/O
 helpers

On Mon, Feb 21, 2022 at 02:04:33PM -0600, Tom Lendacky wrote:
> On 2/18/22 10:17, Kirill A. Shutemov wrote:
> > Port I/O instructions trigger #VE in the TDX environment. In response to
> > the exception, kernel emulates these instructions using hypercalls.
> > 
> > But during early boot, on the decompression stage, it is cumbersome to
> > deal with #VE. It is cleaner to go to hypercalls directly, bypassing #VE
> > handling.
> > 
> > Add a way to hook up alternative port I/O helpers in the boot stub.
> 
> This seems like a lot of churn in order to get this all working without
> taking a #VE.

Well, it evolved from more concise (but also more hacky) implementation:

https://lore.kernel.org/all/20211214150304.62613-11-kirill.shutemov@linux.intel.com

> How cumbersome is it to get #VE handling working in the decompression
> stage? Can you build on any of the support that was added to handle #VC?

We definitely can.

But I still think exception-based implementation is inherently more
fragile. I would rather stick with this.

-- 
 Kirill A. Shutemov